This present invention relates to cooking containers which can be used in microwave ovens, and to methods of using such containers. More particularly, the present invention relates to a container which provides improved microwave heating distributions when used in a microwave oven.
The invention will be particularly described with reference to the microwave cooking of foodstuffs, but it is to be understood that the invention in its broader aspect embraces the provision of containers (and methods of using them) for the microwave heating of bodies of any microwave-heatable material.
Applicant's copending U.S. patent applications Ser. No. 878,171, filed June 25, 1986, and entitled "Microwave Container and Method of Making Same," and Ser. No. 943,563, filed December 18, 1986, and entitled "Microwave Container with Dielectric Structure of Varying Properties and Method of Using Same," the disclosures of which are incorporated herein by this reference, describe containers for containing a material to be heated in a microwave oven. A container as therein described comprises an open topped tray for carrying the material and a lid covering the tray to form a closed cavity, and is characterized in that at least one surface of the container is formed with means for generating a mode of a higher order than that of the fundamental modes of the container, the mode generating means being so dimensioned and positioned with respect to the material when in the container that the mode so generated propagates into the material to thereby locally heat the material. As will be understood, in a container holding a food article being heated in a microwave oven, multiple reflections of radiation within the container or food article give rise to microwave field patterns which can be described as modes. It will also be understood that the term "generating" as used herein embraces both enhancement of modes already existing in the container and superimposition, on existing modes, of modes not otherwise existing in the container.
In a multi-compartment container, such as is used for heating several different foodstuffs simultaneously, the term "container" as used herein should be interpreted as meaning an individual compartment of that container. If, as is commonly the case, a single lid covers all compartments, then "lid" as used above means that portion of the lid which covers the compartment in question.
The container may be made primarily from metallic material, such as aluminum, or primarily from non-metallic material such as one of the various dielectric plastic or paperboard materials currently being used to fabricate microwave containers, or a combination of both.
In a conventional microwave oven, microwave energy, commonly at a frequency of 2.45 GHz, enters the oven cavity and sets up a standing wave pattern in the cavity, this pattern being at fundamental modes dictated by the size and shape of the walls of the oven cavity. In an ideal cavity, only fundamental modes exist, but in practice due to irregularities in the shape of the oven walls, higher order modes are also generated within the cavity and are superimposed on the fundamental modes. Generally speaking, these higher order modes are very weak, and in order to promote better distribution of energy within the container, a "mode stirrer" can be used to deliberately generate or enhance the higher order modes.
If a container, such as a food container, is placed in the microwave oven, and microwave energy is caused to propagate into the interior of that container, then a similar situation exists within the container as exists within the oven itself: a standing wave pattern is set up within the container, this pattern being primarily in the fundamental modes of the container (as distinct from the fundamental modes of the larger oven cavity), but also containing modes higher than those of the fundamental modes of the container, which higher modes are, for example, generated by irregularities in the interior shape of the container and its contents. As before, these higher order modes are generally of much lower power than the fundamental modes and contribute little to the heating of the material within the container.
Attention will now be directed to the manner in which the material within the container is heated by the microwave energy existing within the container. In doing this, it is convenient to study only horizontal planes within the container. It is well known that the standing wave pattern within the container consists of a combined electric and magnetic field. However, the heating effect is obtained only from the electric field and it is therefore of significance to examine the power distribution of the electric field as it exists under steady-state conditions within the container. In the fundamental modes--which, it should be recalled, are those predominantly existing within the container--the pattern of power distribution in the horizontal plane is confined to the edge of the container and this translates into a heating effect which is likewise concentrated around the edge of the container. The material in the central part of the container receives the least energy and therefore, during heating, its center tends to be cool. In conventional containers, this problem of uneven heating is ameliorated by instructing the user to leave the material unattended for a few minutes after the normal microwave cooking time in order for normal thermal conduction within the food to redistribute the heat evenly. Alternatively, the material may be stirred, if it is of a type which is susceptible to such treatment.
The shape of these "cold" areas varies according to the shape of the container. For example, for a rectangular container the shape of the cold area in the horizontal plane is roughly rectangular; for a container which is circular in horizontal cross section, the cold area will be likewise circular and positioned at the center of the container. For an irregularly shaped container, such as is commonly found in compartments of a multi-compartment container, the "cold" area will roughly correspond to the outside contour of the container shape and will be disposed centrally in the container.
In considering the heating effect of higher modes which may or may not exist within the container, it is necessary to notionally subdivide the container into cells, the number and arrangement of these cells depending upon the particular higher order mode under consideration. Each of these cells behaves, from the point of view of microwave power distribution, as if it were itself a container and therefore exhibits a power distribution which is high around the edges of the cell, but low in the center. Because of the physically small size of these cells, heat exchange between adjacent cells during cooking is improved and more even heating of the material results. However, in the normal container, i.e. unmodified by the structures described in the aforementioned copending applications, these higher order modes are either not present at all or, if they are present, are not of sufficient strength to effectively heat the central regions of the food. Thus the primary heating effect is due to the fundamental modes of the container--i.e., a central cold area results.
Recognizing these problems, what the structures described in the aforementioned copending applications seek to do, in essence, is to heat this cold area by introducing heating energy into the cold area. This can be achieved in two ways:
(1) by redistributing the microwave field pattern within the container by enhancing higher order modes which naturally exist anyway within the container due to the boundary conditions set by the physical geometry of the container and its contents, but not at an energy level sufficient to have a substantial heating effect or, where such naturally higher order modes do not exist at all (due to the geometry of the container), to generate such natural modes.
(2) to superimpose or "force" onto the normal field pattern--which, as has been said, is primarily in the fundamental modes--a further higher order field pattern whose characteristics owe nothing to the geometry of the container and whose energy is directed towards the geoxetric center of the container in the horizontal plane which is the area where the heating needs to be enhanced.
In both the above cases, the net result is the same: the container can be notionally considered as having been split into several smaller areas each of which has a heating pattern similar to that of the fundamental modes, as described above. However, because the areas are now physically smaller, normal thermal convection currents within the food have sufficient time, during the relatively short microwave cooking period, to evenly redistribute the heat and thus avoid cold areas. In practice, under certain conditions higher order mode heating may take place due to both of the above mechanisms simultaneously.
The mode generating means described in the aformentioned copending application Ser. No. 878,171 may take one of two forms:
(1) Where said at least one surface of the container takes the form of a sheet of microwave-transparent material, a plate of electrically conductive material which is attached to or forms part of the sheet. Such a plate can be made for example of aluminum foil which is adhered to the sheet, or can be formed as a layer of metallization applied to the sheet.
(2) Where said at least one surface of the container takes the form of a sheet of electrically conductive material, such as aluminum foil, an aperture in the sheet through which microwave energy incident on the sheet can pass. Preferably, the aperture is covered by microwave-transparent material. In some instances, however, the aperture may simply be a void (i.e. open), for example to permit venting of steam from within the container.
It will be appreciated that the two alternatives listed above--i.e., the plate and the aperture--are analogues of one another. For ease of understanding, in the first alternative, the plate can be considered as a two-dimensional antenna, the characteristics of which follow from well-known antenna theory. Thus, the plate can be considered as receiving microwave energy from the oven cavity, whereupon a microwave field pattern is set up in the plate, the characteristics of which pattern are dictated by the size and shape of the plate. The plate then retransmits this energy into the interior of the container as a microwave field pattern. Because the dimensions of the plate are necessarily smaller than those of the container surface with which it is associated, the order of the mode so transmitted into the interior will be higher than the container fundamental modes.
In the second alternative, the aperture can be considered as a slot antenna, the characteristics of which again follow from theory. The slot antenna so formed effectively acts as a window for microwave energy from the oven cavity. The edges of the window define a particular set of boundary conditions which dictate the microwave field pattern which is formed at the aperture and transmitted into the interior of the container. Once again, because the dimensions of the aperture are smaller than those of the container surface with which it is associated, the shape and (particularly) the dimensions of the aperture are such as to generate a mode which is of a higher order than the container fundamental modes.
Several separate higher order mode generating means--be they plates or apertures--may be provided on each container to improve the heat distribution. The higher order mode generating means may all be provided on one surface of the container, or they may be distributed about the container on different surfaces. The exact configuration will depend upon the shape and normal (i.e., unmodified by the plates and/or apertures) heating characteristics, the object always being to get microwave energy into the cold areas, thus electrically subdividing the container down into physically smaller units which can more readily exchange heat by thermal conduction. The considerations which are to be given to the positioning of the higher order mode generating means will depend upon which of the two mechanisms of operation it is desired to use: if it is desired to enhance or generate a particular higher order mode which is natural to the container, then the above-mentioned cell pattern appropriate to that mode should be used to position the plates or apertures forming the higher order mode generating means. In order to enhance or generate a natural mode, a plate/aperture of approximately the same size as the cell will need to be placed over at least some of the cells--the larger the number of cells which have a plate or aperture associated with them, the better the particular mode chosen will be enhanced. In practice, a sufficient space must be left between individual plates/apertures in order to prevent field interaction between them--it is important that each plate/aperture is sufficiently far from its neighbor to be able to act independently. If the spacing is too close, the incident microwave field will simply see the plates/apertures as being continuous and, in these circumstances, the fundamental mode will predominate, which will give, once again, poor heat distribution. A typical minimum spacing between plates would be in the range of 6 to 12 mm, depending upon the particular container geometry and size. A typical minimum spacing between apertures (i.e. where the apertures are separated by regions of foil or other metallized layer) is in the range of 6 to 12 mm., both to protect the electrical integrity of the structure from mechanical damage such as scratches and to avoid ohmic overheating which is likely to result from high induced currents in narrower metal strips; a typical minimum with of metal border regions defining the outer peripheries of apertures would be in the same range, for the same reasons.
If, on the other hand, it is desired to use the mechanism of "forcing" an unnatural higher order mode into the container, then the plate/aperture forming the higher mode generating means needs to be placed over the cold area or areas within the container. In such circumstances, the plate/aperture, in effect, acts as a local heating means and does not (usually) significantly affect the natural modes of the container. Thus the "forced" mechanism utilizes the heating effect of the container fundamental superimposed onto its own heating effect. At certain critical sizes and positioning of the plates, both mechanisms--forced and natural--may come into play.
The aforementioned copending application Ser. No. 943,563 also describes the provision of a microwave heating container characterized in that at least one extended surface of the container is formed with means for modifying the microwave electric field pattern in the container by generating a mode of a higher order than that of the fundamental modes of the container, the modifying means being so dimensioned and positioned with respect to the material when in the container that the mode so generated propagates into the material thereby to locally heat the material. In the container of the latter copending application, however, the modifying means comprises at least a first dielectric wall portion of the container defining a first region of the extended surface and a second dielectric wall portion of the container defining a second region of the extended surface contiguously surrounding the first region, one of these two wall portions having an electrical thickness substantially greater than that of the other.
The latter copending application explains that useful field-modifying or mode generating effects can be achieved with a dielectric (i.e., electrically nonconducting) wall structure by providing appropriately arranged and configured adjacent or contiguous dielectric portions thereof that differ from each other in electrical thickness. For example, referring to those embodiments of structure described in the first-mentioned copending application (Ser. No. 878,171) wherein the extended surface is a sheet of microwave-transparent dielectric material having a conductive metal plate disposed thereon, comparable field-modifying effects are attainable (as set forth in copending application Ser. No. 943,563) by substituting for the metal plate a dielectric portion, in or on the sheet, having a greater electrical thickness than the surrounding portion of the sheet. Again, where in the copending application the higher order mode generating means is a metal sheet defining one or more apertures, in accordance with copending application Ser. No. 943,563 comparable effects are attainable by substituting for the metal sheet an "aperture"-defining dielectric wall portion of relatively high electrical thickness, with the "aperture(s)" constituted of dielectric wall portions of lower electrical thickness. The terms "plate" and "aperture" will be hereinafter sometimes broadly used to embrace the corresponding structures characterized by regions of differing electric thickness, as just described.
In each case, the dielectric wall structure of the invention serves (generally like the metal plate-dielectric sheet or metal aperture-defining sheet structures of the aforementioned copending application Ser. No. 878,171) to establish or generate, within the container, one or more modes of a higher order than the container fundamental mode, so as to achieve a beneficially modified heating distribution in the body of material being heated, as desired (for example) to provide enhanced uniformity of heating throughout the body, or to effect localized intensification of heating in or on selected portions of the body, as for browning or crispening.
The "electrical thickness" of a dielectric wall structure is a function of the actual spatial thickness of the wall (measured, in conventional units of length, between opposed surfaces thereof) and the dielectric constant of the wall material. Stated with reference to microwave energy of a given frequency, having a free-space wavelength W.sub.o, and a wavelength W.sub.m in the dielectric wall material, for a wall having an actual spatial thickness d equal to n.sub.o times the wavelength W.sub.o (d being, of course, also equal to n.sub.m times the wavelength W.sub.m, i.e., d=n.sub.o W.sub.o =n.sub.m W.sub.m) the electrical thickness D may be defined as that spatial distance equal to the number n.sub.m of free space wavelengths W.sub.o, which number n.sub.m =d/W.sub.m. Consequently, EQU D=n.sub.m W.sub.o =d (W.sub.o /W.sub.m)=d (k.sub.m /k.sub.o).sup.1/2,
since W.sub.o /W.sub.m is equal to the square root of the ratio of the dielectric constant k.sub.m of the wall material to the free space dielectric constant k.sub.o. It will therefore be seen that the electrical thickness D of a dielectric wall portion increases with increasing spatial thickness d and/or increasing dielectric constant k.sub.m of the wall portion.
Preferably, in the structures of copending application Ser. No. 943,563, the dielectric wall portion(s) of greater electrical thickness are constituted of material having a higher dielectric constant than the material of the dielectric wall portion(s) of lesser electrical thickness. The portion(s) of greater electrical thickness may also have a greater spatial thickness than the portion(s) of lesser electrical thickness, although this is by no means necessary in all cases. The term "dielectric" herein is to be understood broadly as embracing conventional dielectric (nonconductive) materials and also so-called artificial dielectrics, such as dispersions of metallic particles in a nonconductive matrix, which are characterized by a dielectric constant significantly higher than that of the matrix material alone.
As a further particular feature of the containers of copending application Ser. No. 943,563, one or more of the aforementioned dielectric wall portions may be so constituted as to undergo a change in dielectric constant when subjected to irradiation by microwave energy. In this way, desired changes in heat distribution during the course of heating or cooking may be achieved.
For convenience of explanation, the present discussion considers matters only in the horizontal plane and for the same reason, the only surfaces which are formed with the higher order mode generating means in the embodiments which follow are horizontal surfaces--i.e., the bottom of the container or the lid of the container. However, there is no reason why the teachings of the aforementioned copending applications (and of the present invention) should not be applied to other than horizontal surfaces since the ambient microwave field in which the container is situated is substantially homogeneous.
Because the characteristics of the plate/aperture alternatives are analogous (indeed a particular aperture will transmit an identical mode to that transmitted by a plate of identical size and shape), it is possible to use them interchangeably--in other words, whether a plate or aperture of particular dimensions is used, can be dictated by considerations other than that of generating a particular microwave field pattern.
Clearly, the heating effect of the higher order mode generating means will be greatest in the food immediately adjacent to it and will decrease in the vertical direction. Thus, it may be an advantage to provide higher mode generating means both in the lid and in the bottom of the container. Since the cold areas will be in the same position in the horizontal plane whether the lid or the bottom of the container is being considered, it is clearly convenient to make the higher mode generating means in the lid in registry with those in the bottom of the container. By this means, better heat distribution in the vertical direction can be achieved. It matters not which particular type of higher mode generating means is used as between the lid and the bottom--in one embodiment, for example, a plate or plates are formed on the lid, while in-registry aperture or apertures are formed in the container bottom. In another embodiment, apertures are provided in both lid and bottom surfaces.
Higher-mode generating means such as plates or apertures with peripheries generally conforming to the shape of the container with which they are used (e.g. generally rectangular, in the case of a rectangular container, or circular, in the case of a circular container) have been found highly effective in particular instances in achieving excitation or enhancement of desired higher modes. It has been found, however, that microwave ovens differ significantly from each other in the extent to which these higher modes are generated or enhanced when such mode generating means are employed. Thus, the mode generating means that functions satisfactorily in one oven may produce pronounced local overheating or undercooking in another oven which "feeds" the generated higher mode with greater or less efficiency.
This difficulty has been encountered, for example, in the case of microwave containers of circular horizontal cross section, e.g. containers for pot pies, when the mode-generating means comprises or includes a circular metal foil plate centered on the surface of a microwave-transparent lid of the container or a foil ring mounted on the lid surface in concentric relation to the container periphery. In some ovens, these structures are very satisfactory in obtaining the desired result, viz. that the upper pastry crust be uniformly cooked and browned and that the underlying fill reach uniform temperatures. In other ovens, however, use of the same mode generating means causes either undercooking or overcooking of the central regions of the pie crusts and/or fillings. When simple foil discs or rings are configured to eliminate undercooking of these central regions for some ovens, pronounced overcooking occurs in other ovens; and conversely, discs or rings configured to reduce central region overcooking in these latter ovens cause aggravated undercooking in other ovens. It has thus been difficult to achieve consistently satisfactory heating, with any particular mode generating means, over a wide range of different ovens.